If you had a significant amount of down-force from a spoiler and/or wing, what would you set your spring rates for? The slowest corner or the highest speed straight? With aerodynamic aids the effective sprung weight would vary with speed.

BTW. I use a Bilstein Escort Cup kit torsion bar delete 450f/550r and it is streetable. Car is about 2500-2600lbs.

Tony

 

Hi Tony, in terms of "speed" to calculate for, I would use an average of your cornering speed - that's where you need the lateral traction most. For example, at Watkins Glen, according to my data acquisition, my average speed at a lateral force of 0.5g or greater is 86 MPH. So, in calculating downforce, I'd use that speed.

Now, I'm a little skeptical of your statement about "significant" downforce... I have no wind tunnel data to back up this next statement, but here goes: my gut feeling is that to create significant downforce in a 944, you need a front splitter that's no more than 2" off the pavement an you need a large GT-class style rear wing mounted up high enough to be in the airflow coming off the roof. Certainly there are cars like these at the track, but no one ever calls them "streetable"...

Anyhow, let's go with that amount of downforce for this example.

Again, I need to throw another disclaimer in here: I can't tell you what are the optimum suspension frequencies - because I don't know what they are. But, based on the other people that have posted their spring rates, let's say you'd use a 150 CPM (cycles per minute) without aerodynamic downforce. With the splitter and wing I described, I'd think you'd want to bump that CPM number up to 200 or so.

The next topic for discuss is what should the front/rear frequency ratio be? My research leads me to believe you want a rear frequency that's higher so the rear can "catch up" since the front wheels will hit bumps "first".

If I take your car's weight, 2600 lbs, and assume 54% of the weight is on the front wheels, my calculations show that you'd need a spring rate of 800 lbs/inch in the front to achieve a 190 CPM frequency and a spring rate of 1500 lbs/inch in the rear to achieve a 205 CPM frequency.

Now that I've rambled on a bit, I'm going to tell you that I think those numbers sound high - but, again, I have no experience with spring rates (or suspension frequencies) in these ranges - my gut feeling is that the suspension frequency for a car without aerodynamic downforce should be about 120 CPM. That's much lower than people are generally using. Before everyone flames me, saying "How can 120 CPM be good?! Karl runs 160 CPM and he's faster than you!", understand that I get that number from books - not from real world experience - so I can't vouch for it's accuracy.

If 120 CPM is good, than maybe 150 CPM would be optimal for a car with an aggressive front splitter and a large rear wing. In that case, your 2600 lb car should have 450 lb/in springs in the front and 900 lb/in springs in the rear.

Have I bored you to death yet?